1. Technical Field
The present invention relates generally to an accumulator fuel injection apparatus equipped with a solenoid valve for injecting fuel stored within a common rail (i.e., surge tank) at a high pressure level into an internal combustion engine.
U.S. Pat. No. 4,798,186 to Ganser, issued on Jan. 17, 1989 and U.S. Pat. No. 5,660,368 to De Matthaeis et al., issued on Aug. 26, 1997 disclose electromagnetically controlled fuel injection systems designed to accumulate the fuel within a common rail under pressure through a high-pressure feed pump and inject the fuel into an internal combustion engine. These fuel injection systems use a fuel injector and a solenoid operated two-way valve. The fuel injector includes a pressure control chamber communicating with a high-pressure fuel passage. The two-way valve selectively establishes and blocks fluid communication between the pressure control chamber and a low-pressure chamber to control the fuel pressure acting on a needle valve of the fuel injector for opening and closing a spray hole.
Between the high-pressure fuel passage and the pressure control chamber, a first orifice is formed in a first orifice member to restrict the flow rate of fuel entering the pressure control chamber from the high-pressure fuel passage. A second orifice is also formed in a second orifice member between the pressure control chamber and the low-pressure chamber to restrict the flow rate of fuel flowing from the pressure control chamber to the low-pressure chamber when the solenoid operated two-way valve is opened. When a response rate of the solenoid operated two-way valve is not changed at valve closing and opening, fuel injection characteristics such as injection timing, injection quantity, and rate of injection almost depend upon the flow rate characteristics of the first and second orifices.
Of the fuel injection characteristics, the quantity of fuel at the injection beginning, at the injection end, and during an early part of injection is determined by a difference in flow rate of fuels flowing from the high-pressure fuel passage to the pressure control chamber and flowing from the pressure control chamber to the low-pressure chamber when the solenoid operated two-way valve is opened. The quantity of fuel flowing out of the fuel injector after termination of injection and an interval between a time when the rate of injection shows a peak value and termination of injection (hereinafter, referred to as an injection cut-off period) are determined by the flow rate of fuel flowing from the high-pressure fuel passage to the pressure control chamber after the solenoid operated two-way valve is turned off or closed. Therefore, in order to ensure desired injection characteristics, it is necessary to adjust the flow rate characteristics of the first and second orifices by replacing the first and second orifice plates.
Since the fuel injection characteristics such as the injection timing, the injection quantity, and the rate of injection are, as described above, almost determined based on the flow rate characteristics of the first and second orifices, they will be changed greatly depending upon the shape, sectional area, circularity, inlet dimension, outlet dimension, surface roughness of the first and second orifices.
The optimum fuel injection over a wide range of engine operation which limits the rate of injection at an early part of injection and stops the injection at a high response rate, requires finely drilling the first and second orifices to have a diameter of approximately .phi.0.2 mm to .phi.0.4 mm.
In the De Matthaeis et al. system (U.S. Pat. No. 5,660,368), the first and second orifices are formed in a single injector component. Thus, both the first and second orifices must be replaced even when it is required to change the flow rate characteristics of either of the first and second orifices for adjusting the injection timing and/or the injection characteristics at early and/or late part of injection. This leads to the problem that production yield of injector components for injection characteristic adjustment is decreased. Further, variations in machining accuracy in forming the first and second orifices may mutually affect, thereby making it more difficult to ensure the desired injection characteristics. This also increases the number of times the injector component is replaced until the desired injection characteristics are obtained in an injection characteristics adjustment process.
In the Ganser's system (U.S. Pat. No. 4,798,186), the first and second orifices are formed in different injector components and thus may be replaced separately for changing the flow rate characteristics. One of the injector components having formed therein either of the first and second orifices supports the other slidably. A clearance between sliding surfaces of the injector component pair having formed therein the first and second orifices is decreased as much as possible to facilitate sealing thereof for avoiding leakage of the high-pressure fuel out of the pressure control chamber. Therefore, replacement of only one of the injector component pair may result in an undesirable decrease in the clearance, thereby precluding the sliding motion of the injector components or in great increase in the clearance, thereby leading to the leakage of fuel.